- Aloha Airlines 737 at Maui
- Accident Overview
- Accident Board Findings
- Accident Board Recommendations
- Relevant Regulations / Policy / Background
- Prevailing Cultural / Organizational Factors
- Key Safety Issue(s)
- Safety Assumptions
- Resulting Safety Initiatives
- Airworthiness Directives (ADs) Issued
- Common Themes
- Related Accidents / Incidents
- Lessons Learned
- Aloha Airlines 737 at Maui
Photo of the fuselage damage on Aloha Airlines flight 243
On April 28, 1988, a Boeing 737-200 (line number 152), N73711, was scheduled for several inter-island flights. At the start of the day the first officer performed a normal walkaround pre-flight inspection in the pre-dawn darkness and found nothing unusual. The same crew flew three roundtrip flights from Honolulu to Hilo, Maui, and Kauai. No visual inspections were performed by the flight crew between flights (the airline did not require them). The first officer was replaced at 11:00 for the remainder of the day.
At 13:25 Flight 243 departed from Hilo for Honolulu. As the airplane leveled at 24,000 feet, an explosive decompression occurred. The flight attendant that had been standing at row 5 was immediately swept overboard. The captain took over the controls from the first officer and began an emergency descent to Maui, successfully landing there a short time later.
The airplane had accumulated 89,680 flight-cycles and 35,496 flight-hours at the time of the accident.
There were eight serious injuries (seven passengers and one flight attendant) and one fatality (a flight attendant).
Illustration showing the missing fuselage section.
After the accident, a passenger reported that as she boarded the airplane she had seen a longitudinal fuselage crack (the crack was in the upper row of rivets along the stringer S-10L lap joint, about halfway between the cabin door and the jet bridge hood). She did not report this to the crew before takeoff.
The primary damage to the airplane consisted of the total separation and loss of a major portion of the upper crown skin and other fuselage structure. The damaged area extended from a little aft of the main cabin entrance door aft for about 18 feet. A video, taken from the air just after landing and showing the extent of damage to the airplane is available at the following link (Video taken after flight 243 landed).
The NTSB determined that the probable cause of the accident was the failure of the Aloha Airlines maintenance program to detect the presence of significant disbonding and fatigue damage which ultimately led to the failure of the lap joint at stringer S-10L and the separation of the fuselage upper lobe.
Illustration of a Lap Slice Cold Bonded (Looking Aft)
Lap Splice Design
Adjacent fuselage panels are joined longitudinally by overlapping the edge of the skin of the upper panel about three inches over the edge of the skin of the lower panel (the "lap splice"). On the early 737s (up to line number 291), the overlapping skins were bonded together with an adhesive and were fastened with three rows of rivets. The fuselage pressurization (hoop) loads were intended to be transferred through the adhesive bond, rather than through the rivets.
This design used a cold bond adhesive (a scrim cloth is impregnated with an adhesive that cures at room temperature and must be kept at dry ice temperature until shortly before its use to prevent premature curing). The cold bond process had manufacturing difficulties (surface preparation quality, condensation in the joint during assembly, and premature curing of the adhesive). These difficulties led to the random appearance of bonds with degraded adhesion, with susceptibility to corrosion, and with some areas that did not bond at all. Disbonded areas were then subject to in-service corrosion due to moisture wicking, which leads to further disbonding.
Widespread Fatigue Damage (WFD)
Once disbonding of the lap splice occurs, the fuselage pressurization loads that were intended to be transferred by the adhesive bond are instead transferred by the rivets. Since the countersink for the rivet head went through the entire thickness of the upper skin (creating a knife edge), a higher than typical stress concentration resulted (View flash animation which demonstrates the stress concentration effect).
The combination of effects from the high stress concentration, the rivet load transfer and the far field stress levels led to the development of fatigue cracks at many adjacent or neighboring rivet locations [Multi-Site Damage (MSD)]. An insidious feature of MSD is that many small, hard-to-detect cracks can link up rather suddenly to form a long, critical crack. The advanced stages of MSD, which occurred on this airplane, can result in Widespread Fatigue Damage (WFD), a condition where the airplane structure is no longer able to sustain the required residual strength loads.
Illustration of MSD Cracking Progression
(View Large Image)
Illustration of Redesigned Lap Splice (Typical Section Between Tear-straps)
(View Large Image)
Highlights of Lap Splice Service History
Lap joint corrosion occurring in service was first reported in 1970. Boeing Service Bulletin 737-53-1017 was released later that year to seal the lap joints to prevent corrosion, but this was only partially effective (sealing was also done in production starting at line number 249).
The cold bond lap joint design was discontinued in production in 1972. A redesigned lap joint that had increased joint thickness, allowing the rivets to transfer the pressurization loads, was introduced at line number 292. The adhesive bond that was used for load transfer was eliminated.
Boeing Service Bulletin 737-53-1039 was released in 1972 to inspect for lap splice corrosion.
Starting in 1974 there were reports of tear strap disbonding. The function of tear straps is to arrest the rupture of a crack, allowing for safe decompression by fuselage skin flapping.
Boeing Service Bulletin 737-53-1039 was revised (R2) in 1974 to also inspect for fatigue cracks in the lap splices. At the time of this revision, fatigue cracking had not yet been observed in service.
Boeing Service Bulletin 737-53A1039 was revised (R3) in 1987. The Service Bulletin was upgraded to alert status at this release (by now fatigue cracks had been observed in service at stringers S-4L, S-4R, S-10R, and S-14R).
In 1987 the FAA issued AD 87-21-08 which mandated some of the inspections in Boeing Service Bulletin 737-53A1039. The AD required that only the lap splices at stringers S-4 left and S-4 right be inspected (the service bulletin also provided information to inspect stringers S-10, S-14, S-19, S-20 and S-24, left and right).
The Aloha accident occurred in 1988, following the failure of the lap splice at stringer S-10 left. The accident airplane was line number 152.
Supplemental Structural Inspection Program
In 1978, the portion of 14 CFR 25.571 that addressed fail-safe requirements was amended to add damage-tolerance requirements using fracture mechanics analysis techniques. To address airplanes that were certified before this amendment was effective, a Supplemental Structural Inspection Program (SSIP) was created using damage tolerance concepts. For the 737, the SSIP was to be effective no later than November 1985.
During the creation of the SSIP, airplane structure was classified to determine which structure would be included in the SSIP. The SSIP only included structure where damage detection needed to be achieved through a new, directed inspection. Structure that was deemed to be "damage obvious or malfunction evident" was excluded. The minimum gage fuselage skin was excluded from the SSIP based on an assessment that controlled decompression through flapping would occur, i.e., that it was "damage obvious or malfunction evident."
Maintenance Practices at Aloha Airlines
The Aloha Airlines maintenance program used a D-check (heavy maintenance and inspection check) interval of 15,000 flight-hours, which appears acceptable compared to the 20,000 flight-hour interval recommended by Boeing. However, due to the unusually short flights in the Aloha Airlines flight schedule, flight-cycles were accumulated at about twice the rate that Boeing considered when it produced its maintenance recommendations. In pressurized fuselage structure, the initiation of fatigue cracks and the subsequent rate of crack growth is predominated by the accumulation of flight-cycles, not flight-hours. This fact was not sufficiently regarded when the Aloha Airlines maintenance program was produced and then approved by the FAA (some maintenance tasks should have been more frequent).
The D-check concept, as envisioned in Boeing's Maintenance Planning Document, has each aircraft in the check for a reasonable time period, usually a few weeks. Aloha Airlines split the D-check into 52 separate work packages. The overnight B-checks included portions of D-check items. The NTSB stated that this practice is "an inappropriate way to assess the overall condition of an airplane ..."and further that "...it was obvious to both the maintenance and inspection personnel that each airplane would be needed in a fully operational status to meet the next day's flying schedule."
The post-accident investigation found that the S-4R lap splice had been inspected and repaired as a result of AD 87-21-08. However, a visual inspection of the accident airplane revealed cracks growing from the fastener holes of the top row of rivets. Further analysis showed that these cracks would have been detectable by the eddy current inspections that were required by the AD at the time the AD was accomplished. Aloha Airlines training records revealed that little formal training was provided in NDI techniques and methods. It is not known whether the eddy current inspections were ineffectively performed or if they were accomplished at all as documentation was lacking.
After the accident, visual inspection of the exterior of the airplanes in the Aloha Airlines 737 fleet was conducted. Swelling and bulging of skin, dished fastener heads, pulled or popped rivets, and blistering, scaling, and flaking paint were present at many sites along the lap joints of almost every airplane. According to the NTSB, Aloha Airlines did not produce evidence that it had in place specific severe operating environment corrosion detection and control programs as outlined in the Boeing Commercial Jet Corrosion Prevention Manual. The NTSB noted that "it appears that even when Aloha Airlines personnel observed corrosion in the lap joints and tear straps, the significance of the damage and its criticality to lap joint integrity, tear strap function, and overall airplane airworthiness was not recognized..." It was further noted that "the overall condition of the Aloha Airlines fleet indicated that pilots and line maintenance personnel came to accept the classic signs of on-going corrosion damage as a normal operating condition." After the accident, two aircraft in the remaining Aloha Airlines fleet (N73712 and N73713) were determined to be beyond economical repair and were sold for parts and scrap. The accident airplane, N73711, was also scrapped.